FUNCTIONAL DIFFERENCES BETWEEN THE 2 BELLIES OF THE CRICOTHYROID MUSCLE

The contraction of the cricothyroid (CT) muscle, which results in a de crease in the distance between the thyroid and cricoid cartilages, is considered to be the main factor in lengthening the vocal folds. This is achieved by rotation of the CT joint, The CT muscle is composed of two distinct bellies, the pars recta and the pars obliqua. The functio n of each subunit is not clearly understood, although it is believed t hat they act differently because their fibers run in different directi ons. To clarify the function of the two bellies in phonation, the fund amental frequency (F0), vocal intensity, subglottic pressure, vocal fo ld length, and CT distance were measured using an in vivo canine laryn geal model. On the basis of these measurements, we demonstrated that t he two bellies are varied in their effect on raising the pitch, rotati on, and forward translation of the CT joint. The stimulation of the pa rs recta nerve resulted in a greater increase in the F0 value compared with that of pars obliqua, The combined activity of the pars recta an d pars obliqua is important in adjustment of the vocal fold length. Th e CT approximations directed parallel to the pars recta and pars obliq ua simultaneously were more effective in elevation of the pitch than t he approximation placed parallel to the pars recta only. This finding may be clinically significant with regard to CT approximation thyropla sty in human trails.